For the first time, scientists at New York University and Duke have sequenced the genome of a worm whose lineage has survived 18 million years without having sex. It's one of the oldest known living lineages of an asexual animal, and how it escaped the evolutionary dead end that asexual species normally meet is a mystery.
... Or it was, until its genome was studied by NYU Biology's Helene Fradin, David Fitch, and Karin Kiontke, as well as NYU Abu Dhabi's Fabio Piano and Kristin Gunsalus. Their research, published in Current Biology, found evidence that the tiny worm, Diploscapter pachys, has just one chromosome pair, fused from the six chromosome pairs of a common (sexual) ancestor, and maintains its high genetic diversity by, essentially, cloning itself.
“Scientists have been trying to understand how some animals can survive for millions of years without sex, because such strict, long-term abstinence is very rare in the animal world,” explains Fitch. So how did the asexual D. pachys do it?
A Single Fused Chromosome
Close relatives to D. pachys, like C. elegans, typically have 5-7 chromosomes, and a single chromosome pair is so rare in higher organisms that only two other animal species are known with this condition — an ant and a parasitic roundworm. The researchers decided to sequence the genome of D. pachys to test how the single chromosome was structured, whether by loss or by fusion of multiple ancestral chromosomes. Their results showed that the worm fused the six chromosomes of its ancestor into a single chromosome, and the genealogy showed that this condition likely evolved in the same ancestor as the asexual mode of reproduction.
So the asexual D. pachys evolved from a sexual ancestor, but the question remained on how it had survived for ~18 million years, since evolution is thought to favor sexual reproduction and the genetic diversity it provides.
The End of Gene Recombination
"In the short term, inheriting copies of both parents’ genes usually provides good insurance against mutations that might kill the function of one of those gene copies—a process called complementation,” Fitch says. “In the long term, producing offspring via intercourse allows for adaptation to changing conditions over time because it produces variation through genetic shuffling, or recombination. However, because such shuffling does not occur within asexual species, they tend to go extinct rapidly. So, it has been a longstanding mystery in biology how some asexual animals have survived for so many generations.”
When the research team looked more closely at how D. pachys reproduces, they found that, like many other asexual organisms, the process of making germ cells — sperm or ova — had been modified to prevent recombination, or the reshuffling that results from sexual reproduction. "Basically, the animals were cloning themselves," explains Fitch, by skipping the first stage in meiosis that recombines genes.
So, ironically, D. pachys maintains genetic variation by making sure there is no recombination between the gene copies. “This phenomenon is a significant one in understanding evolutionary genetics," says Fitch, "because it runs counter to the widely accepted view that sexual reproduction is required to eliminate deleterious mutations and for adaptation to a changing environment.”
Other coauthors include researchers from NYU and Duke University. The National Institutes of Health, the Howard Hughes Medical Institute, and the National Science Foundation supported the work. Image courtesy of Karin Kiontke and David Fitch.
"This phenomenon is a significant one in understanding evolutionary genetics, because it runs counter to the widely accepted view that sexual reproduction is required to eliminate deleterious mutations and for adaptation to a changing environment.”